The present invention relates to an exhaust heat recovery type thermal power plant for recovering heat of a gas turbine exhaust gas and, more particularly, to an exhaust recirculation type combined cycle plant for circulating the exhaust of a gas turbine toward the air inlet thereof.
The exhaust heat recovery type thermal power plant is composed of a gas turbine, an exhaust heat recovery boiler for recovering exhaust heat, and a steam turbine driven by steam generated by the exhaust heat recovery boiler, wherein a generator is driven by the gas turbine and the steam turbine to output electricity.
There are various types of the exhaust heat recovery type thermal power plant. As one example, Japanese Patent Laid-Open No. 45924/1989 discloses an exhaust recirculation type thermal power plant for circulating gas turbine exhaust toward the air inlet thereof.
The exhaust heat recovery type combined cycle plant can provide a rapid load variation and has a high efficiency as compared with a normal thermal electric power plant. Therefore, the exhaust heat recovery type combined cycle plant has been rapidly used in recent years, but has some problems when it is operated with a partial load.
One problem is that, at the time of partial load, the thermal efficiency greatly lowers. For example, assume that the thermal efficiency at the rated load is 1, the thermal efficiency at 50% load lowers to about 0.8, and it lowers to about 0.6 at 30% load. The combined cycle plant has the advantage that the load follow ability is higher than that of a thermal electric power plant using a normal boiler. The combined cycle plant is often operated with a variation in load. In this case, however, the plant is operated at the sacrifice of lowering the thermal efficiency thereof, when operated with a partial load.
A further problem is that an output variation on the side of the exhaust heat recovery boiler with a partial load is so great that it is difficult to operate the plant. For example, assuming that the output power of a steam turbine generator at the rated load is 1, it lowers to about 0.43 at 50% load, and it lowers to about 0.12 at 30% load. In addition, the response time of the gas turbine generator (the time from the increase or decrease of the amount of fuel charged into the gas turbine combustor to the change of the generation output of the gas turbine generator) is in the order of several seconds; whereas the response time of the steam turbine generator (the time from the increase or decrease of the amount of fuel charged into the gas turbine combustor to the change of the generation output of the steam turbine generator) is in the order of several minutes. It is therefore very difficult to provide control for obtaining a desired output when the load is changed and immediately thereafter.
With respect to the foregoing, the above-described prior art discloses an arrangement wherein gas turbine exhaust is circulated toward the air inlet thereof, but suggests neither the presence of the aforementioned problems, when operation is carried out at partial load, nor the specific method for solving the problems.